1. Field of the Invention
This invention relates to a thermal mass flow meter including sensors capable of measuring the mass flow rate of liquid or gaseous fluids. A thermal mass flow sensor operates by adding heat and measuring a heat transfer function that is dependent upon the mass flow rate of the fluid through the sensor. Known configurations include 1-, 2- and 3-element types. An example of the 1-element type is the well known hot wire anemometer. The 2-element configuration has one element upstream of the other. Both elements are heated by electric power and cooled by the fluid. The upstream element is cooled by the flow more than the downstream element and the measured temperature difference is a function of fluid mass flow rate. The 3-element configuration has a heated element situated between upstream and downstream temperature-sensing elements. Again, the temperature difference is the measure of flow. We are concerned herein only with 2- and 3- element types.
2. Prior Art
A number of thermal mass flow sensor arrangements have been developed over the past many years. Basically, these have operated on the principle of adding heat energy to a flowing fluid and measuring a heat transfer function and or thermal mass transport function in two sensors spaced in the flowing fluid or on a passageway wall. The measured temperature difference between the upstream and downstream sensors is a function of fluid mass flow. The specific nature of the relationship between mass flow rate and temperature difference is complex, and depends upon fluid properties as well as sensor geometry. The design variables include: sensor style (2- or 3-element); sizes and shapes of heat transfer surfaces; solid thermal conductivities such as those from element-to-element, and from element-to-wall; and flow passage geometry affecting local velocities over the heat transfer surfaces. Fluid properties affecting the flow signal include those dependent upon composition (viscosity, conductivity, specific heat, etc.) but not those dependent upon state (temperature, pressure) because the device is intended to measure mass flow rate independent of state.
An early version of Thomas for large flow applications employed a flow pipe of multi-inch diameter in which two spaced sensor loops were inserted through ports into a circular flow channel with a separate heating element therebetween forming a three-element device. The sensors were made of a material such as platinum, nickel or nichrome which had a temperature-dependent electrical resistivity which varied over a moderate temperature range. When the fluid at one temperature having passed by the first upstream sensor is then heated to a higher temperature, the resistivity of the downstream second sensor is changed, the measured temperature difference between the sensors being the measure of flow. The temperature rise in the gas is a function of the amount of heat added, the sensor geometry and conductivity, the mass flow rate and the properties of the gas.
More recently, a two-element mass flow meter and controller has been developed by Tylan Corp. of Carson, Calif. in which a pair of equally heated upstream and downstream resistance sensors made of Nichrome wire are externally wound around a sensor flow tube. When fluid (gas) is flowing in the tube, heat is transferred along the line of flow from the upstream to downstream thermometers/sensors producing a signal proportional to the gas flow. The higher the flow, the greater the differential between the sensors. Each sensor forms part of a bridge and amplifier circuit that produces a zero to 5 V DC signal proportional to gas flow. This signal is compared to a command voltage from a potentiometer or the like and an error signal generated which can adjust a valve to change the gas flow until a preset set point is reached. The sensor flow tube is in this device approximately 0.010" ID.times.a length greater than 1" which will handle flow rates of only a few standard cc's per minute. Higher flow rates are accommodated by passing additional gas flow around the sensor through a flow splitter designed to maintain a constant and known bypass ratio.
A mass flow controller sold by Brooks Instruments, Hatfield, Pa., utilizes a horizontal bypass sensor tube with upstream and downstream sensor coils exterior of the tube and a heater element similarly wound between the sensors on the tube exterior. A bridge detects the temperature difference caused by the greater heat input to the downstream sensor and an amplifier provides an output to control circuitry. The sensor tubes have an internal diameter of from 0.010" to 0.060" inches. In each of the last two devices, heat conduction is through the tube wall resulting in relatively slow long response times, i.e., several seconds. Such devices also generally require heating of the fluid to 100.degree.-200.degree. C. greater than the ambient of the incoming fluid to give satisfactory performance. In many gaseous applications, this may be above the safe temperature limit of the gas or cause decomposition of the gas or reaction with contaminants. Further, for each gas composition and flow range, the device must be calibrated because of nonlinearities and inconsistent correction factors.